-We obtain, using extensive Monte Carlo simulations, virial expansion and a highdensity perturbation expansion about the fully packed monodispersed phase, the phase diagram of a system of bidispersed hard rods on a square lattice. We show numerically that when the length of the longer rods is 7, two continuous transitions may exist as the density of the longer rods in increased, keeping the density of shorter rods fixed: first from a low-density isotropic phase to a nematic phase, and second from the nematic to a high-density isotropic phase. The difference between the critical densities of the two transitions decreases to zero at a critical density of the shorter rods such that the fully packed phase is disordered for any composition. When both the rod lengths are larger than 6, we observe the existence of two transitions along the fully packed line as the composition is varied. Low-density virial expansion, truncated at second virial coefficient, reproduces features of the first transition. By developing a high-density perturbation expansion, we show that when one of the rods is long enough, there will be at least two isotropic-nematic transitions along the fully packed line as the composition is varied. and adsorbed gas molecules on metal surfaces [11][12][13][14][15]. A system of hard sphero-cylinders in three-dimensional continuum undergoes a transition from an isotropic phase to an orientationally ordered nematic phase as density is increased. Further increase in density leads to a smectic phase with partial translational order and a solid phase [10,[16][17][18][19][20]. In two-dimensional continuum, a Kosterlitz-Thouless transition to a high-density phase with power law correlations may be observed [21][22][23][24]. Lattice models of hard rods, of interest to this paper, also have a rich phase diagram in two dimensions, while not much is known in three dimensions.